Select the option in which the pair of words is not

Solution

The logic followed here is as follows: The second word is the synonym of the first word. In Abundant: Plentiful, both words mean "existing in large quantities." In option a: Drowsy: Lethargic are synonyms (both mean sleepy/tired). In option b: Candid: Honest are synonyms (both mean truthful/frank). In option d: Vague: Obscure are synonyms (both mean unclear). In the word pair 'Fragile: Sturdy', 'Sturdy' is the antonym of 'Fragile'. So, 'Fragile: Sturdy' is the pair of words which are not related in the same way as the pair of words Abundant: Plentiful.